1. Field of the Invention
This invention relates to a positioner, and more particularly to an improvement of a feed positioner in which, for example, a feed screw and a feed nut engaged therewith are used to linearly move a driven part coupled with the feed nut in accordance with the rotation of the feed screw.
2. Description of the Related Art
Heretofore, this type of feed positioner has been widely adopted for machine tools, measuring instruments, or the like as a device for moving an object linearly.
The feed positioner comprises a feed screw having a male thread and a feed nut having a female thread, in which the relative rotation between the feed screw and feed nut which are engaged with each other is converted into the linear movement in the axial direction of the feed screw.
The feed screw and feed nut are designed and machined so that a small clearance will be produced at a point of engagement, which is intended to minimize a variation in torque resulting from an error in machining precision or a change in dimensions due to a temperature change. A small backlash therefore occurs in a feeding direction in the feed positioner.
As a result, in the feed positioner, when the feed screw and feed nut make a relative rotation, a movement corresponding to the backlash is made in the axial direction of the feed screw irrelevant of the rotation. When rotation is reversed, immobilization may occur. Otherwise, when external force is applied in a direction in which a backlash occurs, a stop position may change or a feeding speed may change.
A method for avoiding the backlash (disclosed in Japanese Patent Lied Open No. 60-196462) is such that: one or more slots are formed to extend in the axial direction of the female thread of the feed nut; the presence of the slot or slots helps reduce the rigidity in radial direction of the female thread; and spaces between adjoining slots are narrowed by applying force to the outer circumference of the female thread, so that the male and female threads are pressurized. Thus, occurrence of a backlash is prevented.
Another method (disclosed in Japanese Patent Publication No. 61-59413) is such that: a female thread of a feed nut is divided into two sections or a feed nut has two female threads; one of two female-thread sections or female threads is separated from a feed nut support; a detent is formed; and thus a clearance between two female-thread sections or two female threads is changed a little without causing them to make a relative rotation, so that a plane between the male thread and the female-thread sections or female threads is pressurized. Thus, occurrence of a backlash is prevented.
In the above conventional methods, when a feed screw is engaged with a feed nut, a clearance for a backlash or an extent of pressurization must be adjusted.
After a prolonged use, male and female threads have worn down to change their dimensions. This results in a widened clearance between the male and female threads. Consequently, a back lash occurs between the male and female threads or the extent of pressurization decreases. This leads to deterioration of performance with the passing of time. Consequently, re-adjustment or other maintenance becomes necessary, which is a nuisance.
In another method, a clearance between male and female threads is substantially removed using a part of relatively high rigidity; such as, a tapered member, a screw, or a spacer, so that pressurization is effected. A change in dimensions due to a temperature change or an error in machining precision between male or female threads causes the clearance or extent of pressurization to change. This results in a variation in friction and torque, whereby a torsion of, for example, the axis of the male thread changes. Consequently, an error occurs in the precision of a feeding speed or a stop position.
In yet another method, the elasticity of an elastic body such as a spring is used to press a female thread to a male thread, and thus a clearance between the male and female threads is removed to pressurize the male and female threads. Herein, an extent of pressurization can somewhat be varied depending on a change in dimensions of the male or female thread. A variation in friction torque can therefore be reduced to some extent. However, this method makes it necessary to minimize the rigidity of the female thread. When the elasticity of an elastic body is increased in order to reduce the rigidity of the female thread to a level attained in the foregoing position restriction method, a variation in force relative to deformation of the elastic body becomes more intense. As a result, the extent of pressurization varies, whereby the friction torque varies. Consequently, an error occurs in the precision of a feeding speed or a stop position.